FIG. 2 is a cross-sectional view of a buried ridge semiconductor laser diode (hereinafter referred to as an ES-LD) with an etch stopping layer (hereinafter referred to as an ES layer). This laser has an n-type GaAs substrate 1 as a substrate. An n-type Al.sub.0.5 Ga.sub.0.5 As cladding layer 2 is disposed on the n-type GaAs substrate 1 and a multi-quantum well (hereinafter referred to as MQW) structure active layer 3 comprising GaAs and Al.sub.0.2 Ga.sub.0.8 As is disposed on the n-type cladding layer 2. A p-type Al.sub.0.5 Ga.sub.0.5 As first cladding layer 4 and an AlGaAs etch stopping layer 5 having an Al composition ratio above 0.6 are successively disposed on the MQW active layer 3. A p-type Al.sub.0.5 Ga.sub.0.5 As second cladding layer 6 in a ridge shape 8 together with a p-type GaAs contact layer 7 on the layer 6 and n-type current blocking layers 9 at both sides of the ridge 8 and including Zn diffused regions 10 containing Zn as a p-type dopant are disposed on the etch stopping layer 5. An n side electrode 15 is on the rear surface of the n-type GaAs substrate 1 and a p side electrode 16 is on the p-type GaAs contact layer 7 and the Zn diffused regions 10 at the front surface of the device.
To operate the semiconductor laser, a forward direction voltage is applied between the p side electrode 16 and the n side electrode 15, electrons are injected from the n-type GaAs substrate 1, and holes are injected from the p-type cap layer (contact layer) 7. The injected holes are confined to the central portion of the element by the n-type GaAs current blocking layers 9 and the electrons and holes efficiently recombine in the active layer 3 directly below the ridge 8, thus emitting light having a wavelength corresponding to the energy band gap of the active layer 3. In this prior art device, the emitted light has a wavelength of 780 nm. The light generated directly below the ridge 8 tends to be broadened along the active layer 3 in the horizontal direction. Because of the light absorption due to the current blocking layers 9 disposed in the vicinity of the active layer 3, there is an effective refractive index different in the active layer 3 in the horizontal direction so that light is confined within the ridge 8. This light confinement in the width direction largely depends on the thickness of the p-type Al.sub.0.5 Ga.sub.0.5 As first cladding layer 4. The best device characteristics are obtained when the thickness of the first cladding layer 4 is 0.2-0.3 .mu.m.
FIGS. 3(a)-3(d) illustrate a process for forming the ridge of the ES-LD shown in FIG. 2. The same reference numerals are used to designate the same elements as those in FIG. 2.
As shown in FIG. 3(a), a metal organic chemical vapor deposition (MOCVD) process is used to grow the n-type AlGaAs cladding layer 2, the active layer 3, the p-type AlGaAs first cladding layer 4, the AlGaAs etch stopping layer 5, the p-type AlGaAs second cladding layer 6, and the p-type GaAs contact layer 7 successively and epitaxially on the n-type GaAs substrate 1.
Thereafter, an insulating film, comprising SiN or SiO, is formed on the wafer by sputtering, thermal CVD, or plasma CVD, photoresist is deposited on the insulating film, and photolithography and etching are performed to form a stripe-shaped insulating film 11 having a width corresponding to the width of a ridge that is produced later.
Next, as shown in FIG. 3(c), portions of the contact layer 7 and portions of the second cladding layer 6, other than at the ridge, are removed by etching employing the insulating film 11 as an etching mask. Etching is stopped by the etch stopping layer 5 by employing an etchant that does not etch an AlGaAs layer having an Al composition ratio above 0.6 but that does etch an AlGaAs layer having an Al composition ratio below 0.6. For example, a solution of tartaric acid and hydrogen peroxide can be employed. Thus, it is possible to keep the thickness of the p-type first cladding layer 4 constant, significantly affecting and providing preferable device characteristics.
Thereafter, the n-type GaAs current blocking layers 9 at both sides of the ridge 8 are epitaxially grown, burying the ridge 8 (FIG. 3(d)).
Finally, the Zn diffused regions 10 are formed at upper portions of the current blocking layers 9 by ion implantation employing the film 11 as a mask. The insulating film is then removed, completing the device. The Zn diffused regions 10 may also be performed by solid phase diffusion.
In the prior art ES-LD constructed as described above, the current blocking layer 9 is disposed on the etch stopping layer 5 having an Al composition ratio above 0.6. However, since that AlGaAs etch stopping layer has a high Al composition ratio, it has a thick surface oxide film. As a result, layers epitaxially grown in that surface have a density of surface defects in excess of 10.sup.6 /cm.sup.2, thereby significantly deteriorating surface morphology. In addition, a leakage current flows through the surface defects and makes no contribution to the device operation. The leakage current increases with time, significantly reducing device reliability.